EFFECT OF AMBIENT AND EVOLVED CO2 ON AN EQUILIBRIUM REACTION IN YBA2CU3O6+X SYNTHESIS

The formation of YBa2Cu3O6+x in air involves two main reactions. The f irst is between BaCO3 and CuO to form BaCuO2 and CO2. Increasing the p artial pressure of carbon dioxide in air shifts the endothermic BaCO3 reaction to higher temperatures. At a fixed carbon-dioxide concentrati on in air, the reaction onset-temperature of a thick sample (2 mm) det ermined by TG/DTA exceeds by approximately 200 degrees C that of a thi n film (10 mm) determined by in situ HT-XRD. Evolved CO2 in thick samp les disguises the dependence of the intrinsic reaction onset-temperatu re and reaction rate on the ambient CO2 concentrations. The intrinsic equilibrium constant and associated thermodynamic parameters for this reaction should be determined by experiments in which the impact of tr ansport limitations is eliminated, i.e., use of a thin film and high g as-velocity next to the sample surface.